Granules

The polyethylene for our films is produced from oil.

 

Depending on its country of origin, crude oil ranges from light to dark brown and thin to glutinous.

 

Even the composition varies, although crude oil essentially always consists of a mix of many hydrocarbons. These have a variety of boiling points.

 

In refineries, this crude oil is heated (cracked) and the bonds in the crude oil are separated from each other. A very small portion of this - the hydrocarbon gas - goes to the chemical industry, the majority of the oil is refined into fuel oil and petrol.

 

Along with other thermoplastics - e.g. polypropylene - polyethylene is a polyolefin. Gaseous ethylene, an unsaturated hydrocarbon, is the olefin with the simplest structure and the basic component of polyethylene:


H H
| |
C=C
| |
H H Fig.1

 

As carbon is always tetravalent, bonding is possible either with more C atoms or with atoms of other elements. A hydrocarbon is said to be unsaturated if only two bonds (see figure 1) or even only one bond is occupied by H (hydrogen) atoms. These unsaturated hydrocarbon bonds are highly reactive. The C = C double bonds of the ethylene molecules are broken under pressure and linked together (the resultant free bonds take up more and more new ethylene molecules). This results in the formation of long, chain-like macromolecules from equal components.

 

 


H H H H H H
| | | | | |
- C - C - C - C - C - C -
| | | | | |
H H H H H H Fig.2

Polymerisation can be seen as a type of cooking process in a pressure cooker. Polyethylene, a solid plastic, comes from ethylene gas. The polyethylene molecules contain branches. Both the level of molecule chain branching and the length of the side chains have a significant effect on the properties of the polyethylene. This is also where the reasoning for the differences between LDPE and HDPE can be found. HDPE is produced by plymerisation under high temperatures and low pressure. LDPE is exactly the opposite - here, polymerisation takes place at low temperatures but high pressure. This allows longer and more branched molecule chains to form with HDPE than with LDPE. As a result, has a higher density than LDPE, which has a fairly low density. This also explains the origin of the names: HDPE = High Density Polyethylene, LDPE = Low Density Polyethylene. Thus, HDPE can never be a high pressure polyethylene, which is, however, easy to confuse. It is also somewhat confusing. Polyethylene manufacturers (e.g. BASF, Degussa-Hüls or Bayer) generally sell and supply polyethylene - whether HDPE or LDPE - in granulate form.

 
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